It would probably be smarter to hire a local HVAC pro to come out a balance your system with a hood. They'll have much better equipment than you can purchase which will be much more accurate. Why would you want to spend money on something you're only going to use once or twice? Let the people who know what they're doing come out, you'll be much better off.
A ball shapped cat toy works great to illustrate the imbalance behind closed doors that have a small undercut. Get one with a bell in it and you can have audio as well as visual.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!